The invention relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals, where the field thereof has a significant component parallel to the liquid-crystal layer, containing a liquid-crystalline medium having positive dielectric anisotropy, where the medium comprises at least one mesogenic compound which contains a group of the formula I* 
in which
L1 is H or F.
In conventional liquid-crystal displays (TN, STN, OMI and AMD-TN), the electric fields for realignment are produced essentially perpendicular to the liquid-crystal layer.
International Patent Application WO 91/10936 discloses a liquid-crystal display in which the electric signals are produced in such a way that the electric fields have a component, suitably a significant parallel component, to the liquid-crystal layer (IPS, in-plane switching). The principles of operating such a display are described, for example, by R. A. Soref in Journal of Applied Physics, Vol. 45, No. 12, pp. 5466-5468 (1974).
EP 0 588 568 discloses various ways of addressing such a display.
These IPS displays can be operated with liquid-crystalline materials either having a positive or a negative dielectric anisotropy (xcex94xcex5xe2x89xa00). However, with the materials known hitherto, IPS displays have relatively high threshold voltages and long response times. The object was therefore to indicate liquid-crystalline materials which are suitable for achieving relatively low threshold voltages and short response times in IPS displays.
Surprisingly, this object has been achieved by using liquid-crystalline materials which comprise at least one compound which contains a laterally fluorinated 4-(cyclohexylcarbonyloxy)benzonitrile group.
Such compounds are disclosed, for example, in JP 58-210 057 and JP 4-279 560.
However, these specifications give no indication that these substances can be used to improve the threshold voltages and the response times of IPS displays.
The invention thus relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals, where the field thereof has a significant component parallel to the liquid-crystal layer, comprising a liquid-crystalline medium having positive dielectric anisotropy, where the medium comprises at least one mesogenic compound which contains a group of the formula I* 
in which
L1 is H or F.
Suitable mesogenic compounds typically include (but are not limited to) those having 2-4 ring groups connected in series by bridge groups, single bonds or combinations thereof, wherein terminal ring groups exhibit wing groups. Suitable ring groups include a 1,4-cyclohexenylene or trans-1,4-cyclohexylene radicals in which, in addition, one or more non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94Sxe2x80x94; 1,4-phenylene radicals in which,in addition, one or two CH groups may be replaced by N; or radicals from the group consisting of 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl. Ring groups, preferably 1,4-cyclohexylene or 1,4-phenylene, may be substituted, e.g., by one or more fluorine atoms.
Suitable bridge groups include, but are not limited to,:
xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1C xe2x80x94, a single bond, xe2x80x94(CH2)4xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CH2CH2xe2x80x94.
Suitable wing groups include:
H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, up to perhalo, wherein addition one or more CH2 groups in these radicals may each, independently of one another be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94 xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Cxe2x80x94 in such a way that O atoms are not linked directly to one another.
The compound containing the group I* is suitably present in an amount of, e.g., 5-30%, preferably 7-20% by weight, although suitable amounts beyond these ranges are possible and routinely determinable.
Preferred embodiments are IPS displays in which
a) the medium comprises at least one compound of the formula II 
in which
R1 is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted by halogen, up to perhalo, where, in addition, one or more CH2 groups in these radicals may each, independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another,
Q is a radical of the formula
xe2x80x94(O)qxe2x80x94(CH2)rxe2x80x94(CF2)sxe2x80x94
in which
q is 0 or 1,
r is 0 or an integer between 1 and 6, and
s is an integer between 0 and 6,
X is F or Cl, and, in the case where sxe2x89xa00, is alternatively H, and
xe2x80x83A1 and A2 are each, independently of one another,
(a) a 1,4-cyclohexenylene or trans-1,4-cyclo-hexylene radical in which, in addition, one or more non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94Sxe2x80x94,
(b) a 1,4-phenylene radical in which, in addition, one or two CH groups may be replaced by N,
(c) a radical from the group consisting of 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
xe2x80x83where the radicals (a) and (b) may be substituted by one or two fluorine atoms,
xe2x80x83L2 and L3 are each, independently of one another, H or F.
Z1 and Z2 are each, independently of one another, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond, or one of the radicals Z1 and Z2 is alternatively xe2x80x94(CH2)4xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94CH2CH2xe2x80x94, and
m is 0, 1 or 2;
b) the medium comprises at least one compound of the formula I 
xe2x80x83in which
L1 is H or F, and
R2 is as defined for R1;
c) the medium has a dielectric anisotropy xcex94xcex5 of  greater than 8.5, preferably between 8.6 and 14, in particular between 8.7 and 13.5;
d) the medium comprises at least one compound of the formula III
R3xe2x80x94(A3xe2x80x94Z3)0xe2x80x94A4xe2x80x94R4xe2x80x83xe2x80x83III
xe2x80x83in which
R3 and R4 are each, independently of one another, as defined for R1,
A3 and A4 are each, independently of one another, as defined for A1 and A2,
Z3, is in each case independently of the others, as defined for Z1 and Z2 and,
O is 1, 2 or 3; or
e) the medium comprises at least one compound of the formula IV 
xe2x80x83in which
R5 is as defined for R1, 
Z is a single bond or xe2x80x94COOxe2x80x94, and
L4 and L5 are each F or H,
but not of the formula I.
Particular preference is given to an IPS display in which the pixels are addressed by means of an active matrix.
The invention furthermore relates to a liquid-crystalline medium having positive dielectric anisotropy which comprises at least one compound of the formula II and at least one compound containing a group of the formula I*, e.g., a compound of formula I, in particular which preferably comprises, e.g.,
from about 60 to 95% by weight, more preferably from 70 to 90% by weight, of at least one compound of the formula II,
from about 5 to 30% by weight, more preferably from 7 to 20% by weight, of at least one compound of the formula I,
from about 5 to 20% by weight, more preferably from 7 to 15% by weight, of at least one compound of the formula III,
from about 1 to 10% by weight, more preferably from 2 to 8% by weight, of at least one compound of the formula IV.
The medium preferably comprises at least one compound selected from the formulae IIa, IIb and IIc: 
xe2x80x83at least one compound selected from the formulae Ia and Ib: 
xe2x80x83if desired at: least one compound selected from the formulae IIIa, IIIb and IIIc: 
xe2x80x83and
xe2x80x83at least one compound selected from the formulae IVa to IVe: 
xe2x80x83and
xe2x80x83Z2 is xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond, and R1, R2, R3, R4, R5, L1 to L5 are as defined in each case.
The novel liquid-crystalline media generally have a birefringence (xcex94n) of  less than 0.12, preferably between 0.07 and 0.1, in particular between 0.075 and 0.09.
The flow viscosity (at 20xc2x0 C.) of the novel materials is generally less than 30 mm2 sxe2x88x921, in particular between 15 and 25 mm2xc2x7sxe2x88x921. The rotational viscosity of the novel media is generally less than 200 mPaxc2x7s, preferably less than 150 mPaxc2x7s, in particular between 80 and 120 mPaxc2x7s. The resistivity of the novel materials is generally between 5xc3x971010 and 5xc3x971013 xcexa9xc2x7cmxe2x88x921 at 20xc2x0 C.
It has been found that even a relatively small proportion of the cyclohexanecarbonyloxybenzonitrile compounds mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formula II, III and/or IV, results in a significant reduction in the threshold voltage and in fast response times, and at the same time broad nematic phases having low smectic-nematic transition temperatures are observed. The compounds of the formulae I to IV are colorless, stable and readily miscible with one another and with other liquid-crystal materials.
Suitable xe2x80x9calkylxe2x80x9d moieties are, but not limited to, straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atoms are generally preferred.
Suitable xe2x80x9calkenylxe2x80x9d moieties are, but not limited to, straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Exemplary alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
Alkyl-substituted by halogen, i.e., xe2x80x9chaloalkylxe2x80x9d, preferably covers straight-chain groups containing terminal halogen, preferably fluorine, i.e., fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.
Alkyl in which one or more CH2 groups are replaced by xe2x80x94Oxe2x80x94, i.e., xe2x80x9coxaalkylxe2x80x9d preferably covers straight-chain radicals; of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m, in which n and m are each, independently of one another, from i to 6. n is preferably 1 and m is preferably from 1 to 6.
Through a suitable choice of the meanings of R1 to R5, the addressing times, the threshold voltage, the steepness of the transmission characteristic lines, etc., can be modified as desired. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally result in shorter addressing times, improved nematic tendencies and a higher ratio between the elastic constants k3, (bend) and k11 (splay) compared with alkyl and alkoxy radicals. 4-alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and smaller values of k33/k11 compared with alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group in Z1 or Z2 generally results in higher values of k33/k11 compared with a single covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission characteristic lines in TN cells with a 90xc2x0 twist (in order to achieve grey shades) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexability), and vice versa.
The optimum mixing ratio between the compounds of the formulae I and II+III+IV depends substantially on the desired properties, on the choice of the components of the formulae I, II, III and/or IV and on the choice of further optional components. Suitable mixing ratios within the above range can easily be determined from case to case.
The total amount of compounds of the formulae I to IV in the novel mixtures is not crucial. The mixtures can therefore contain one or more further components in order to optimize the various properties. However, the observed effect on the addressing times and on the threshold voltage is generally greater the higher the total concentration of compounds of the formulae I and II.
In a particularly preferred embodiment, the novel media comprise compounds of the formula II in which Qxe2x80x94X is F, OCF3 or OCHF2. A favorable synergistic action with the compounds of the formula I results in particularly favorable properties.
The novel liquid-crystalline media preferably comprise 2 to 40, in particular 4 to 30, components as further constituents besides one or more compounds of the formulae I, II, III and IV. These media very particularly preferably contain 7 to 25 components besides one or more compounds according to the invention. These further constituents are preferably selected from nematic or nematogenic (monotropic or isotropic) substances, in particular substances from the classes of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl esters of cyclohexanecarboxylic acid, phenyl or cyclohexyl esters of cyclohexylbenzoic acid, phenyl or cyclohexyl esters of cyclohexyl-cyclohexanecarboxylic acid, cyclohexylphenyl esters of benzoic acid, of cyclohexanecarboxylic acid and of cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, cyclohexylcyclohexylcyclohexenes, 1,4-bis-cyclohexylbenzenes, 4,4.xe2x80x2-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2-cyclohexylethanes, 1-cyclohexyl-2-(4-phenylcyclo hexyl)ethanes, 1-cyclohexyl-2-biphenylylethanes, 1-phenyl-2-cyclohexylphenylethanes optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids. The 1,4-phenylene groups in these compounds may also be fluorinated.
The most important compounds suitable as further constituents of novel media can be characterized by the formulae 1, 2, 3, 4 and 5:
Rxe2x80x2xe2x80x94Lxe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x831
Rxe2x80x2xe2x80x94Lxe2x80x94COOxe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x832
Rxe2x80x2xe2x80x94Lxe2x80x94OOCxe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x833
Rxe2x80x2xe2x80x94Lxe2x80x94CH2CH2xe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x834
Rxe2x80x2xe2x80x94Lxe2x80x94Cxe2x89xa1Cxe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x835
In the formulae 1, 2, 3, 4 and 5, Rxe2x80x2 and Rxe2x80x3 are each independently selected from one of the definitions of R1. L and E, which may be identical or different, are in each case, independently of one another, a bivalent radical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc is trans-1,4-cyclo-hexylene or 1,4-cyclohexylene, Pyr is pyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl and G is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.
One of the radicals L and E is preferably Cyc, Phe or Pyr. E is preferably Cyc, Phe or Phexe2x80x94Cyc. The novel media preferably contain one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which L and E are selected from the group comprising Cyc, Phe and Pyr and simultaneously one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which one of the radicals L and E is selected from the group comprising Cyc, Phe and Pyr and the other radical is selected from the group comprising -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the compounds of the formulae 1, 2, 3, 4 and 5 in which the radicals L and E are selected from the group comprising -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.
In a smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5, Rxe2x80x2 and Rxe2x80x3 are in each case, independently of one another, alkyl, alkenyl, alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy having up to 8 carbon atoms. This smaller sub-group is called group A below, and the compounds are labelled with the sub-formulae 1a, 2a, 3a, 4a and 5a. In most of these compounds, Rxe2x80x2 and Rxe2x80x3 are different from one another, one of these radicals usually being alkyl, alkenyl, alkoxy or alkoxyalkyl.
In another smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5 which is known as group B, Rxe2x80x3 is xe2x80x94F, xe2x80x94Cl, xe2x80x94NCS or xe2x80x94(O)iCH3-(k+1)FkCl1, where i is 0 or 1, and k+l is 1, 2 or 3; the compounds in which Rxe2x80x3 has this meaning are labelled with the sub-formulae 1b, 2b, 3b, 4b and 5b. Particular preference is given to those compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b in which Rxe2x80x3 is xe2x80x94F, xe2x80x94Cl, xe2x80x94NCS, xe2x80x94CF3, xe2x80x94OCHF2 or xe2x80x94OCF3.
In the compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b, Rxe2x80x2 is as defined for the compounds of the sub-formulae 1a-5a and is preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.
In a further smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and 5, Rxe2x80x3 is xe2x80x94CN; this sub-group is known as group C below, and the compounds of this sub-group are correspondingly described by sub-formulae 1c, 2c, 3c, 4c and 5c. In the compounds of the sub-formulae 1c, 2c, 3c, 4c and 5c, Rxe2x80x2 is as defined for the compounds of the sub-formulae 1a-5a and is preferably alkyl, alkoxy or alkenyl.
In addition to the preferred compounds of groups A, B and C, other compounds of the formulae 1, 2, 3, 4 and 5 having other variants of the proposed substituents are also customary All these substances can be obtained by methods which are known from the literature or analogously thereto.
Besides novel compounds of the formula I, the media according to the invention preferably comprise one or more compounds selected from group A and/or group B and/or group C. The proportions by weight of the compounds from these groups in the novel media are preferably
Group A: 0 to 90%, preferably 20 to 90%; in particular 30 to 90%
Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65%
Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%,
the sum of the proportions by weight of the group A and/or B and/or C compounds present in the particular media according to the invention preferably being 5% to 90% and in particular 10% to 90%.
The novel media preferably comprise 1 to 40%, particularly preferably 5 to 30%, of compounds according to the invention. Further preferred media are those which comprise more than 40%, in particular 45 to 90%, of compounds according to the invention. The media preferably comprise three, four or five compounds according to the invention.
The structure of the novel IPS display corresponds to the construction which is conventional for such displays, as described, for example, in WO 91/10936 or EP 0 588 568. The term conventional construction here is broadly drawn and also covers all derivatives and modifications of the IPS display, in particular, for example, also matrix display elements based on poly-Si, TFT or MIM.
However, an important difference between the novel displays and the conventional ones is in the choice of the liquid-crystal parameters of the liquid-crystal layer.
The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in the lesser amount is dissolved in the components making up the principal constituent, expediently at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and, after thorough mixing, to remove the solvent again, for example by distillation.
The dielectrics may also contain further additives known to the person skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopants can be added.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German application No. 195 37 802.4, filed Oct. 11, 1995, is hereby incorporated by reference.
In the following discussion, C denotes a crystalline phase, S denotes a smectic phase, SB denotes a smectic B phase, N denotes a nematic phase and I denotes the isotropic phase.
V0 denotes the voltage for 0% a transmission (viewing direction perpendicular to the plate surface). ton denotes the switch-on time and toff denotes the switch-off time. Vmax denotes the voltage at maximum transmission. xcex94n denotes the optical anisotropy and n0 denotes the refractive index (in each case 589 nm). xcex94xcex5 denotes the dielectric anisotropy (xcex94xcex5=xcex5∥xe2x88x92xcex5xe2x8axa5, where xcex5∥ denotes the dielectric constant parallel to the long axes of the molecules and xcex5xe2x8axa5 denotes the dielectric constant perpendicular thereto). The electro-optical data were, measured in an IPS cell at 20xc2x0 C., unless expressly stated otherwise. The optical data were measured at 20 C. unless expressly stated otherwise.
An IPS test cell as described in WO 91/10936 having a comb structure was used, where the electrodes were 20 xcexcm apart.
The layer thickness d of the liquid-crystal material is 5 xcexcm. The cell furthermore has:
The cells; are dark in the xe2x80x9coffxe2x80x9d state.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by acronyms, the transformation into chemical formulae taking place as in Tables A and B below. All the radicals CnH2n+1 are straight-chain alkyl radicals containing n carbon atoms. The coding in Table B requires no further explanation. In Table A, only the acronym for the parent structure is given. In individual cases, a code follows for the substituents R1, R2, L1 and L2, separated from the acronym for the parent structure by a hyphen: